1. Field of the Invention
The present invention relates to antennas. More specifically, the present invention relates to system and methods for forming beams and creating nulls using phased array antennas.
2. Description of the Related Art
Adaptive antenna systems have been developed to perform beam forming or spatial nulling. With knowledge of the direction of the signal source, conventional antenna beam forming techniques endeavor to maximize the signal to noise ratio with respect to signals sent to or received from desired sources and attempt to steer nulls in the direction of undesirable sources.
Unfortunately, in many cases it may be difficult to ascertain the direction of the signal source with sufficient accuracy. This is particularly problematic with respect to spread spectrum and other signals having a signal strength below the noise level.
Conventional beam forming techniques require knowledge of the direction of the signal sources and a method to track the angle of arrival of the signal on a moving platform. Two methods are generally employed to acquire knowledge of the direction of the signal source of interest: angle of arrival approaches and adaptive searching for the signal direction.
In the angle arrival approach, a receiver estimates the angle arrival of the desired signal and performs adaptive signal processing to maximize the gain of the beam in the pointing direction. With this approach, assumptions must be made with respect to the relative location of the signal source. However, for many applications, an assumption with respect to the location of the signal source may introduce an unacceptable amount of error into the process.
On a moving platform, an initial measurement unit (IMU) is required to maintain the desired pointing direction. This solution can be expensive and potentially require an IMU of considerable size and weight.
Further, in a dynamic environment, the signal sources may move around requiring a communication of a large amount of data from one platform to another. Hence, angle of arrival approaches tend to be expensive, cumbersome and prone to error.
In electronic warfare applications, adaptive searching is often used to identify the location of a source of a jamming signal. The searching is typically performed by sweeping a radar receiver in azimuth and/or elevation. Unfortunately, the efficacy of this approach is limited in situations where the jamming source is intermittently activated.
Hence, a need existed in the art for a more effective, less-expensive system or method for ascertaining the direction of a signal source relative to conventional approaches. The need was addressed by the teachings of U.S. patent application Ser. No. 09/891,686, entitled SYSTEM AND METHOD FOR FORMING A BEAM AND CREATING NULLS WITH AN ADAPTIVE ARRAY ANTENNA USING ORTHOGONAL EIGEN-WEIGHTING filed Jun. 26, 2001 by K. V. Cai, the teachings of which are incorporated herein by reference.
Unfortunately, as is well-known in the art, the capacity of an array is dictated by the number of antenna elements thereof. For example, an antenna with four radiating elements, has three degrees of freedom. That is, the antenna has the capability of forming three beams or two beams and one null or one beam and two nulls or three nulls. In general, a phased array antenna has a degree of freedom equal to the number of radiating elements minus one. When the degrees of freedom of an antenna are dedicated to interference cancellation, the antenna has no remaining capability to perform other functions. Hence, in environments with many jammers, the antenna may be preoccupied with interference cancellation.
U.S. Pat. No. 5,271,038, issued Dec. 14, 1993, to K. V. Cai and entitled DISTORTION SUPPRESSION USING THRESHOLDING TECHNIQUES, the teachings of which are hereby incorporated herein by reference, discloses and claims a method for detecting a interferer using excision in the spectral domain. However, this technique is directed primarily to the elimination of the signal from a narrowband interferer. Inasmuch as many interferers may be wideband interferers, operating over a broad frequency range, this technique alone may be inadequate for many applications.
Hence, a need remains in the art for a system and method for eliminating narrow and wideband interferers while leaving the antenna with at least one degree of freedom to perform other functions.
The need in the art is addressed by the system and method for forming a beam and creating nulls of the present invention. The inventive method combines frequency domain suppression technology for narrow-band jammer rejection with orthogonal weighting using Eigen analysis for jammer nulling and signal beam-forming for an array antenna system. The inventive system is adapted for use with an adaptive array antenna having a plurality of radiating elements. The system includes a frequency domain suppressor connected to the array antenna and an orthogonal weighting subsystem.
In the illustrative implementation, signals received by each radiating elements are processed by an analog front-end and digitized. The frequency domain suppressor includes a Fast Fourier transformer (FFT) and a frequency domain suppressor for each element in the array. The frequency domain suppressor further includes a threshold evaluator which reads the signals output by the FFT""s and provides threshold controls to the spectral suppressors.
The orthogonal weighting subsystem includes an adaptive weighting unit connected to the output of each of the spectral suppressors. The orthogonal weighting subsystem further includes an estimator for determining and optimal orthogonal Eigen-weighting to be applied to each of the adaptive weighting units. The outputs of each of the adaptive weighting processors are combined and inverse Fast Fourier transformed to provide one or more output beams and or one or more nulls in an efficient manner designed to leave the system with at least one degree of freedom for other functions.
The invention combines narrowband interference suppression with adaptive antenna nulling to improve anti-jamming capability. It provides a systematic method to excise partial band jammers prior to antenna nulling to maintain high degree of freedom for antenna nulling. It provides a spectral nulling technique which is less sensitive to partial band jamming tactics (i.e., rapid change in amplitude, frequency) that may cause degradation with conventional adaptive nulling techniques. The inventive technique can also be used to sort different emitters in frequency, amplitude and angle arrival (AOA), and can be used to characterize signals. And the inventive technique provides signal isolation from interference in spatial domain to support the Multiple Access capability (i.e., Spatial Domain Multiple Access or SDMA).